They may be flying drones, in particular rotary-wing drones such as helicopters, quadricopters and the like. But the invention is however not limited to the piloting of and the data exchange with flying devices; it applies as well to rolling devices progressing on the ground under the control of a remote operator, the term “drone” having of course to be understood in its most general meaning.
A typical example of flying drone is the AR.Drone 2.0 or the Bebop (registered trademarks) from Parrot SA, Paris, France, which are quadricopters equipped with a series of sensors (accelerometers, gyrometers, altimeters), a front video camera capturing an image of the scene towards which the drone is directed, and a vertical-view camera capturing an image of the overflown ground. Another type of drone to which the invention may apply is the Jumping Sumo, also from Parrot SA, which is a remote-controlled rolling and jumping toy provided with accelerometer and gyrometer sensors and with a front video camera.
The front video camera can be used for an “immersive mode” piloting of the drone, i.e. where the operator uses the image of the camera in the same way as if he were himself on board the drone. It may also serve to capture sequences of images of a scene towards which the drone is directed, the operator using the drone in the same way as a camera that, instead of being held in hand, would be borne by the drone. The collected images can be recorded, put online on web sites, sent to other Internet users, shared on social networks, etc.
The WO 2010/061099 A2, EP 2 364 757 A1 and EP 2 450 862 A1 (Parrot) describe the principle of piloting a drone through a touch-screen multimedia telephone or tablet having integrated accelerometers, for example a smartphone of the iPhone type or a tablet of the iPad type (registered trademarks).
Hereinafter, the term “tablet” will be generally used to refer to this device, but this term must not be understood in its narrow meaning; quite the contrary, it also includes the functionally equivalent devices, in particular all the portable devices provided with at least one visual display screen and wireless data exchange means, such as smartphone, multimedia player with no phone functions, game console, etc.
The tablet incorporates the various control elements required for the detection of the piloting commands and the bidirectional exchange of data via a radio link of the Wi-Fi (IEEE 802.11) or Bluetooth wireless local network type directly established with the drone. Its touch screen displays the image captured by the front camera of the drone, with, in superimposition, a certain number of symbols allowing the control of the flight and the activation of commands by simple contact of the users finger on this touch screen.
The bidirectional wireless radio link comprises an uplink (from the tablet to the drone) and a downlink (from the drone to the tablet) to transmit data frames containing:                (from the tablet to the drone) the piloting commands, hereinafter simply called “commands”, sent at regular intervals and on a systematic basis;        (from the drone to the tablet) the video flow coming from the camera; and        (from the drone to the tablet) as needed, flight data established by the drone or state indicators such as: level of the batteries, flight phase (take-off, automatic stabilisation, landed on the ground, etc.), altitude, detected fault, etc.        
The invention more precisely relates to the bidirectional transmission, with an increased range, of these commands and data between the drone and the tablet.
Indeed, a remote control via a direct Wi-Fi radio link, as described in the above-mentioned EP 2 450 862 A1, has a limited range, of at most about fifty meters, which corresponds to piloting a drone that remains in direct view of the operator.
If a greater range is desired, a first solution consists in using a Wi-Fi range extender (repeater), which is a transceiver acting as a relay and provided with a power amplifier allowing to increase the power radiated on the radio channel used between the tablet and the receiver. The range can hence be increased up to 200 meters.
This technique has however a drawback, due to the very high variability of the binary rate or bitrate of the radio link (such bitrate being often called the “bandwidth”). This bitrate depends in particular on the distance between the drone and the tablet, and on the presence or not of obstacles liable to more or less disturb the transmission of the radio waves. When the drone moves away from the tablet, the available bitrate deteriorates rapidly, which will cause a conflict between the uplink flow (sending of the commands) and the downlink flow (essentially the video flow).
Indeed, an important parameter to be considered is the very high need in video bitrate of the downlink, very higher than that required for the sending of the flight commands and data. Typically, the required video bandwidth is, even after compression of the images, of the order of 2.5 to 3 Mbits per second (Mbps), to be compared to a few kilobits per second (kbps) for the sending of the flight commands and data, which are not simple numerical values. In other words, the video occupies typically more than 95% of the available bitrate.
That way, the bitrate reduction will have an impact on the bitrate allocated to the commands by the uplink, hence it will follow a deterioration of the quality of transmission of the uplink, with a risk of sporadic loss of frames containing the commands produced by the tablet, and for consequence a control of the drone becoming very difficult.
To remedy this drawback, it is possible to provide two different links, one for the video and the other for the flight commands and data: in this case, the video return of the drone towards the tablet is operated by a Wi-Fi link with relay by the Wi-Fi repeater, whereas the flight commands and data are exchanged with a dedicated remote-control device (box provided with joysticks, cursors, buttons, etc.) via a radio link adapted to the exchange of the only flight commands and data with the drone, this link being distinct from the Wi-Fi link of transmission of the video flow.
This solution (that will be exposed in more details hereinafter with reference to FIG. 2) remedies to the risk of conflict between the video flow and the flight command and data flow. On the contrary, it has three major drawbacks:                the use of a repeater involves a time share of the channel between the drone and the tablet, which limits on each side (tablet/repeater and repeater/drone) the bitrate to 50% of what would be obtained with a direct Wi-Fi link between the tablet and the drone;        the repeater being an equipment of the symmetrical type, if it transmits a powerful signal towards the drone, it will also transmit a powerful signal towards the tablet, with a risk of saturation because the latter is far closer to the repeater than the drone;        if a high range is desired, it is necessary to use a non-standard Wi-Fi protocol, in particular to take into account a high latency due to the distance. The modification of the protocol must be applied to the whole link, both on the tablet/repeater side and on the repeater/drone side, so that the Wi-Fi that must be implemented by the tablet will no longer be a standard Wi-Fi.        
The DE 10 2012 012 462 A1 describes a configuration comprising a remote-control console, a tablet incorporated to the console, and a remote-control model. Each of these three elements is provided with a processor and a communication interface incorporating in particular a wireless communication module of the Bluetooth type. The configuration may further be supplemented by a signal converter interfaced (wirelessly or through a cable) to the console, as well as to the tablet and to the model. This converter serves as a communication hub between these three elements. The model may hence be indifferently remote-controlled from the console or from the tablet, which then plays the role of an auxiliary or emergency remote control with respect to the main remote control formed by the console. Hence, in case of failure of the console or the tablet, the user still has an operational remote control that allows it to safely bring back the model to the right place.